When properly designed and implemented, automobile air bag restraint devices can dramatically reduce the injury and loss of life caused by automobile collisions. A properly deployed air bag may cushion a driver or passenger, thereby reducing the risk of injury. Proper air bag deployment includes rapidly inflating the air bag to a volume sufficient to cushion the occupant. Air bag inflator elements and their design and operation are described in the commonly owned and copending application for Method and System for Evaluating Gas Generants and Gas Generators, Attorney Docket No. 1090.2.105, which is incorporated herein by reference.
Gas bags are typically inflated principally by combustion of a chemical gas generant, but the gas generant may also be used in conjunction with other inflator elements. As the gas generant combusts, it produces gases which rapidly increase both the temperature and pressure within the air bag. The inflation must be rapid, so that the inflated air bag is in place in time to protect the occupant. Inflation typically occurs within about ten milliseconds of the time the gas generant begins combusting.
The temperature of the inflating gas flow is a significant design factor for several reasons. The gas is separated from the car's occupant only by the inflated air bag, and is also typically vented into the car's interior as the air bag deflates after use. Thus, the temperature must be low enough to avoid burning the car's occupants. The air bag must also be pressurized sufficiently to absorb the impact of an occupant. But adjustments in pressure may cause changes in temperature, and vice versa according to well-known physical laws, so both temperature and pressure readings are useful in designing inflators.
Gas temperatures may be measured with a thermocouple. A thermocouple is a temperature sensor that includes two different metals joined together at a junction. The junction produces a small thermoelectric voltage when the junction is heated. The metals may be electrically connected to a thermocouple thermometer that interprets the change in voltage as a change in temperature. A thermocouple placed in a housing for use as a temperature probe is known as a thermocouple probe.
Two methods are known for increasing the rapidity with which a thermocouple responds to changes in temperature. First, the junction between the two metals may be exposed to the ambient environment. Protected junctions are less subject to corrosion or structural stresses, but respond more slowly than exposed junctions.
Second, the amount of each metal used may be reduced. For instance, thermocouples are typically made by joining two wires at a junction and electrically connecting the other end of each wire to the thermocouple thermometer. Smaller wires have faster response times.
Exposed junction thermocouples having small wires may respond quickly enough to be useful in measuring temperatures at millisecond intervals, or at similar intervals of interest in evaluating gas bag inflators. However, small unprotected wires do not stand up well to the gas flows present in gas bag generators caused by combustion of gas generants as the generator pressure increases from about 100 pounds per square inch to as high as about 2500 pounds per square inch. A gas flow may tear the junction apart or tear the junction off of the thermocouple probe. Corrosive compounds from the gas flow may be deposited on the exposed junction.
Whether the junction is exposed or not, additional stresses are often presented when the probe is inserted into or removed from a typical gas bag inflator test apparatus. The probe is disposed with the junction located inside a closed container that holds the gas generant. The other end of the probe, which is connectable to the thermocouple thermometer, is positioned out side the container. The probe is clamped about its circumference at a location between these two ends to obtain an air-tight seal that prevents gas flow from escaping the gas generator. Small unprotected wires in the probe are easily damaged when the probe is moved or when it is clamped.
Thermocouples may be strengthened by enclosing the junction or by using larger wires, but either approach substantially increases the thermocouple's response time. If the response time is too great, the probe becomes useless. For instance, a probe with a response time measured in tenths of a second has little value in measuring gas flow temperatures at one-millisecond intervals.
Thus, it would be an advancement in the art to provide a rugged thermocouple that responds rapidly to temperature changes.
It would also be an advancement to provide such a thermocouple having a junction that is sufficiently rugged to withstand repeated exposure to the gas flow in a gas bag inflator.
It would be an additional advancement to provide such a thermocouple having a junction that responds to temperature changes rapidly enough to measure the gas flow temperature at a plurality of distinct times during a gas bag inflator test.
It would be a further advancement in the art to provide such a thermocouple which is sufficiently rugged to survive normal handling in a lab during gas bag inflator tests without losing effectiveness as a measuring instrument.
Such a thermocouple is disclosed and claimed herein.